Process for selectively anodically dissolving copper from zirconium



United States Patent Office 2 Claims. (61. 204-32 The present invention, relates to a process for the selective anodic dissolution of a metal coating.

It is common practice for a metal article or an intermediate product which is sensitive to the air to be surrounded by a continuous sealed sheath or covering, before being subjected to shaping operations. This protective covering protects the central part against changes due to the air, at temperatures to which it is taken either during deformation to which it is subjected by forging, drawing or laminating or during annealing operations which precede deformation operations carried out in the cold or between such operations.

It is also advantageous to provide such a covering to insulate such a part when it is desired to facilitate lubrication, during deformation steps, in order to avoid any contamination due to friction between the machined surfaces.

Elimination of the protective covering, when the deformation operations are completed, can be effected 'by mechanical means, but most often this elimination is either very costly or incomplete, because of intermetallic penetration which can take place between the covering and the article itself.

It is preferable, therefore, to make use of chemical dissolution, but the use of the active reagents currently utilised has certain disadvantages:

(a) Production of vapours of fumes, which require complicated and troublesome trapping arrangements and neutralisations;

(b) Partial dissolution of the metal of the covering, the reagent eliminating the products of interpenetration due to interdiffusion phenomena between the two metals;

(c) If the covering does not have uniform thickness, in general, dissolution of the article begins when it has become bared and continues until the metal of the covering has completely disappeared;

(d) In the case where the protected part is constituted by a metal of high cost, dissolution of a certain quantity of this metal involves an exorbitant loss as such dissolution can present certain dangers which absolutely prohibit it.

The process of selective anodic dissolution of a metallic coating accordingto the invention does not have the disadvantages mentioned above. In essence, the protected part with its metallic covering is treated electrolytically as an anode, by utilising a rapid-action electrolyte of a composition which is substantially independent of the duration of the operation of the dissolution, the metal which is to be uncovered not being attacked by the electrolyte or being covered by a barrier layer under selected conditions for the transfer of the covering metal.

According to the invention, a selective electrolytic dissolution process is provided, which comprises anodising an article comprising zirconium or a zirconium-base alloy having a covering of copper or a copper-base alloy in an electrolyte containing copper ions and sulphate ions.

Preferably, the electrolyte contains 25 to 100 g./l. of sulphuric acid and 50 to 100 g./l. of copper sulphate.

,In a preferred embodiment of the process of the invention, the electrolysis potential is from 0.5 to v., the

3,334,029 Patented Aug. 1, 1967 cathodic current density is from 0.2 to 15 a./sq. dm. and the cathode-to-anode surface ratio is from 0.1 to 10.

The process of the invention has several advantages:

(1) It is rapid;

(2) It is practical, because the anodic dissolution stops automatically over all regions where the article has been uncovered, as the metal of the covering fixed to the article by superficial ditfusion is dissolved;

(3) The electrolyte does not evolve any fumes during or after its period of activity;

(4) The action of the electrolyte is constant, even when the protective metal has not been uncovered;

(5) It is economical, because it does not require frequent bath changes and only causes a very slight dissolution of the bared metal.

A preferred embodiment of the process according to the invention is described below by way of example, it being understood that this embodiment is given only by way of example.

The example relates to the anodic dissolution of a copper covering plate-d on to an article of zirconium or an alloy of zirconium with other metals such as uranium. The copper covering, which has been applied to the article by a hot extrusion operation, followed if desired by working in the cold or at moderate temperature and annealing in air, is to be removed so that the article itself can be subjected to finishing operations which impart to it the dimensional tolerances and surface state desired.

At present, dissolution of the covering is carried out chemically in fluoro-nitric baths or nitric baths which cause the evolution of noxious vapours which must be trapped, neutrilsed and scrubbed. The bath becomes loaded with copper salts and zirconium salts, which necessitates an expensive chemical recovery process.

The application of the invention to this particular case consists in effecting the dissolution of the copper covering by anodising, viz, electrolysis as an anode, under the following conditions.

The metallic part surrounded by its covering which is to be dissolved away constitutes the anode of the electrolytic cell used. The cathode is constituted by grids of wires or expanded metal located above and below the anode at a minimum distance of 5 mm.

The electrolyte is a solution of 100 g. per litre of copper sulphate containing 50 g. per litre of sulphuric acid at 66 B. (sp. gr.=1.83).

The temperature of the electrolysis bath should be from 2030 C.

The potential applied between the anode and the cathode is maintained at a value of from 0.5 to 6 volts, the cathodic current density being from 0.5 to 1 a./dm.

in the case where the bath is not agitated.

Under these conditions, the speed of dissolution of the copper of the covering constituting the anode is about 1 g. per amp. per hour, the copper being transferred to the cathode where it deposits in a compact form. The thickness of the copper covering diminishes by mm. per lamp. per drn. (current density) per hour.

When the zirconium alloy article comes into contact with the electrolyte, an anodic oxide layer forms which has a thickness of less than 0.1 micron at the temperature and potential and in the electrolyte indicated. It is the same if the anode is of zirconium. The copper which has diffused at the covering/article interface is entirely.

dissolved. Even in the case where electrolysis is carried out for a long time after complete dissolution of the copper, the electrolyte only contains low quantities of zirconiurn and there is zirconium alloyed with copp r in the interditfusion layer which may be formed.

If the article is of a zirconium-uranium alloy, the manium content of which is less than 6%, the same electrolyte even after prolonged operation only contains very small quantities of uranium, corresponding to an interdiifusion layer formed between the copper and zirconiumuranium alloy.

The operation of electrolysis does not involve the evolution of fumes or vapours. When the surface of the article has oxide points with sharp corners or plates which are more or less encrusted, these can become detached :at the end of the dissolution. They fall to the bottom of the bath without becoming dissolved in the electrolyte.

The electrolyte does not undergo any modification of composition and is always ready for use and requires virtually the minimum of maintenance. The electrolyte can be agitated if desired or not. The copper deposited on the cathode can be recovered directly in the metallic state.

The process which has been described can be applied to metallic parts which have various different shapes. If the process is applied to an article of great length, this is placed in an electrolytic bath, rolled in a spirial with spacers giving distances of 10-20 mm. between adjacent turns When the article coated with the covering has a tubular or hollow form, an auxiliary cathode is used which penerta-tes into the re-entrant part.

What we claim is:

1. In a selective electrolytic dissolution process, the step of anodising an article of zirconium or a zirconiumbase alloy having a substantially continuous covering of copper or a copper-base alloy in an electrolyte solution of copper ions and sulphate ions having 25-100 g./l. of sulphuric acid and from -200 g./l. of copper sulphate, in which the electrolysis potential is from 0.5 to 10 v., in which the cathodic current density is from 0.2 to 15 a./dm. and in which the cathode-to-anode surface ratio is from 0.1 to 10.

2. In a selective electrolytic dissolution process, the step of anodising an article of zirconium or a zirconiumbase alloy having a substantially continuous covering of copper or a copper-base alloy in an electrolyte solution of copper ions and sulphate ions having 50 g./ l. of sulphuric acid and g./l. of copper sulphate, in which the electrolysis potential is from 0.5 to 6 v., in which the current-density is from 0.5 to l a./dm. in which the electrolysis temperature is from 20 to 30 C. and in which the cathode-to-anode surface ratio is from 0.1 to 10.

References Cited UNITED STATES PATENTS 2,196,133 4/1940 Webb i 204l46 3,063,917 11/1962 Barto 204l46 X OTHER REFERENCES Miller, Zirconium, Academic Press, Inc., publishers, New York, 1954, page 203.

JOHN H. MACK, Primary Examiner.

ROBERT K. MIHALEK, Examiner. 

2. IN A SELECTIVE ELECTROLYTIC DISSOLUTION PROCESS, THE STEP OF ANODISING AN ARTICLE OF ZIRCONIUM OR A ZIRCONIUMBASE ALLOY HAVING A SUBSTANTIALLY CONTINOUS COVERING OF COPPER OR A COPPER-BASE ALLOY IN AN ELECTRLYTE SOLUTION OF COPPER IONS AND SULPHATE IONS HAVING 50 G./1. OF SULPHURIC ACID AND 100 G./1. OF COPPER SULPHATE, IN WHICH THE ELECTROLYSIS POTENTIAL IS FROM 0.5 TO 6 V., IN WHICH THE CURRENT-DENSITY IS FROM 0.5 TO 1 A./DM.2 IN WHICH THE ELECTROLYSIS TEMPERATURE IS FROM 20* TO 30*C. AND IN WHICH THE CATHODE-TO-ANODE SURFACE RATIO IS FROM 0.1 TO
 10. 